Most conventional drying methods and apparatus, such as laundry tumble dryers, employ air as the medium for transporting sensible heat energy from a heat source into the moisture in the material being dried in order first to heat and then to evaporate the moisture, and then to carry away the steam produced by that evaporation. Such methods and apparatus are inefficient in that typically up to around 75% of the sensible energy drawn from the heat source is wasted, mainly by emission of hot air to atmosphere, while the partly sensible but mainly latent energy in the steam produced by the evaporation of the moisture is not recovered as this too is emitted hot to atmosphere.
In such known drying methods and apparatus the kinetic energy required to provide the fan power to blow the air over the heat source and through the material being dried is finally dissipated and thus also wasted by emission to atmosphere as additional sensible energy within the steam and the air carrying it away. Thermal insulation of the apparatus is also generally inadequate for preventing significant loss of heat.
In known forms of apparatus heat exchange and air recirculation methods and apparatus are sometimes employed to reduce the wastage of sensible energy within the hot emission to atmosphere. For example, there may be provision for cross flow heat exchange between the hot air and steam emission and the cool ambient air being drawn towards the heat source to make use of part of the sensible energy which is otherwise wasted. Also, recirculation of a proportion of the hot air and steam emission back over the heat source may be employed to reduce the volume of cool ambient air required and thus reduce the amount of sensible energy to be transferred from the heat source.
Probably the best of the known recirculation methods typically reduces the total amount of sensible energy transferred from the heat source by around 40% by reducing the sensible energy wastage referred to earlier (typically up to about 75%) by around 50%. Other known recirculation or heat exchange methods tend to provide smaller wastage reductions.
Because the dew point temperature of the emission, even when the best of the known recirculation methods is employed, seldom exceeds 40.degree. to 50.degree. C., useful recovery of the latent energy from the steam contained within the emission, for example by condensation in a condenser to heat water, cannot viably be effected.
Another known form of drying apparatus sets out to achieve 100% air recirculation and thereby eliminate the need to emit air and steam through ducting. In such apparatus the steam contained within what would otherwise be the hot air and steam emission is condensed out of the recirculating air flow onto a coil cooled by a continuous flow of cold water passing through it. At the same time the heat from the circulating air is transferred to the water and both the resulting condensate and the then only slightly warmed water flow to drain, carrying with them virtually all of the thus wasted energy input. Such arrangement is commonly used in domestic tumble dryers. In commercial circumstances, however, where mains water is charged for by volume and not by a fixed annual Domestic Water Rate, or where means of cooling the water would need to be provided, such a method of avoiding the need for emission ducting and providing a flow of only slightly warmed water would not be viable.